In order to facilitate electrical connections among semiconductor device terminals and external leads, wirebonds are formed by coupling a bondwire to a bonding site. Generally a bondwire is positioned at a bonding site, such as a bond pad on a chip or PCB, while various forces are applied to cause the metals of the wire and bond pad to fuse. The bondwire is usually made of gold, although bondwires of other metals such as aluminum or copper are also used in the semiconductor manufacturing industry. Ball bonding is a common wirebond process. For the purpose of providing context and example, gold ball bonding is discussed herein, although the invention may also be practiced with ball bonding using other materials. During gold ball bonding, a generally spherical or other shape gold nodule, frequently called a “ball” is formed by melting the end of a gold bondwire held by a part of a bonding tool known as a capillary. Capillaries known in the art are typically ceramic axially symmetrical tools with vertical feedholes for feeding the bondwire through the center. Capillaries commonly hold and control the bondwire during the bonding process and are used to exert pressure and transmit ultrasonic energy to the bonding location.
The ball is brought into contact with the bond pad. A controlled amount of pressure is then applied by the capillary for a selected amount of time, contributing to the formation of a metallurgical weld between the bondwire and the bond pad as well as deforming the ball into its final shape. The application of pressure interacts with the application of ultrasonic energy as further discussed below. The wirebonding tool severs the wire in preparation for the next wirebond by clamping the wire and raising the capillary. Problems are sometimes encountered in the application of pressure to form the bond. If the pressure is insufficient, a poor bond can result. Excessive pressure can result in damage to associated circuitry.
Wirebonding requires that the bondwire and bond pad be placed in intimate contact with each other. Better bonds are achieved when contact is enhanced using ultrasonic energy. The ultrasonic energy is applied as the ball at the end of the bondwire is brought into contact with the bond pad by the capillary, exerting pressure to push the ball against the bond pad. The ultrasonic energy applied by the bonding tool abrades the ball of the bondwire against the surface of the bond pad. This abrading action cleans the bond pad of debris and oxides that may be present, exposing a fresh surface of the bond pad useful for forming a secure bond. The continued application of ultrasonic energy also results in mutual deformation of the bondwire and bond pad where they abrade against one another, which can enhance the metallurgical bond. The amount of ultrasonic energy applied must be carefully controlled. Excessive ultrasonic energy can stress the nascent bond, and the application of insufficient ultrasonic energy fails to adequately enhance bonding. The application of insufficient pressure can also inhibit the effective transmission of ultrasonic energy.
When the bondwire and bond pad are made from different metals, such as in the case of wirebonding gold wire to an aluminum bond pad, for example, thermal energy is used to enhance bonding. In effect, heat is used to soften the harder metal, in this example aluminum, to match the hardness of the softer metal, in this example gold. In order to reach the required temperature, heat is applied to the bond pad by heating the underlying surface, such as a PCB. Insufficient heat can inhibit bonding, while excessive heat can result in damage to the circuitry. Some configurations, such as overhangs or stacked die assemblies present particular problems in directing an acceptable amount of heat to the bonding site.
Due to these and other problems, it is desirable to avoid unnecessary heating of devices during wirebonding, to efficiently direct thermal energy to desired locations, and to avoid excessive ultrasonic vibrations. Improved wirebonding techniques and systems adapted to provide one or more of these or similar benefits would be useful and advantageous in the arts.